In oil and gas wells which have been drilled and have casing installed there is generally a string of production tubing installed in the casing of the well. The fluid or gas from the producing formation travels up through this production tubing and is controlled by valves on the wellhead. In many conventional wells this tubing string is made from lengths of steel tubing screwed together to form a long string of production tubing. In many other wells, a coil of tubing generally described in the industry as coiled tubing is utilized. This tubing arrives at the well site in a large coil and is unrolled and inserted in the well with the appropriate equipment. Coiled tubing requires no screwed joints and results in considerable savings where it can be utilized. Both types of tubing generally require that they be held in position in the wellhead by a piece of equipment called a dognut. The upper end of the production tubing string attaches to the dognut and is positioned and locked into a tubing spool in the wellhead with special screws called dog screws or dogs. This operation is called landing the dognut and production tubing in the tubing spool.
In conventional wells, the dognut and production tubing landing operation is quite well known and requires that the last length of production tubing installed be of the right dimension so that the dognut will land in the correct place in the tubing spool. The upper end of the production tubing is generally held in the dognut by being screwed into the bottom of the dognut or an upset at the top end of the tubing is screwed onto the adapter of what is called a wrap-around dognut. This technology is standard practice.
In wells where coiled tubing is installed there is a problem with the termination of the coiled tubing in the dognut. When the coiled tubing is in place in the well it must be cut off and a thread cut on the exposed end (or some type of expanded portion made on this end of the coiled tubing, sometimes by welding) to hold the tubing in the dognut. The dognut is then landed in the tubing spool. The coiled tubing, although straightened by the coiled tubing injector mechanism, is not perfectly straight nor round and this leads to alignment problems when cutting and threading the coiled tubing and when landing the dognut. This problem is also evident when re-attaching to the tubing at later dates. Many times this work will be done with the well casing under pressure, adding to the complexity of the problems.